Pre-payment television system using a memory card associated with a decoder

Claim 1

A pay-television system using a memory card associated with a decoder, wherein, on an emission side, a digital generator (10) supplies a random code (XA) which is sent to an input of a main card (11) which enciphers said random code, by using a predetermined key (100), and the enciphered code is included in a broadcasted video signal, a first device (17) has inputs and receives at its inputs, on the one hand, said random code (XA) and, on the other hand, the code (XB) enciphered by the main card (11) and carries out a determined digit combination of those two codes, this combination forming the initialization word of a pseudo-random generator (12) which is intended to supply a code table designed to scramble the video signal, and on the reception side, the decoder receives the enciphered code (XB) that is deciphered by a subscriber card (13), a second device (18) has inputs and receives at its inputs, on the one hand, the enciphered code (XB) and, on the other hand, the code deciphered (XB) by the subscriber card (13) and carries out a digital combination of those two codes, this combination being identical to the one of the first device (17), the result of this combination constituting the initialization word for another pseudo-random generator (14) supplying a code table identical to the one of the emission, this code table being designed to unscramble the video signal.